1. Field of the Invention
The present invention relates to an image forming apparatus that is usable in a color printer, a color copying machine, etc., capable of forming images in color components through a plurality of image forming stations and further, capable of forming a color image by superimposing these images on a paper.
2. Description of the Related Art
As an image forming apparatus, a color copying machine that has an endless running conveyer belt and a plurality of image forming stations in different colors provided in parallel with the conveyer belt is known. In this copying machine, a paper is adsorbed on the conveyer belt and an image is formed on that paper by superimposing images in different colors through color image forming stations.
In this kind of color copying machine, it is regarded extremely difficult to bring the running speed of a conveyer belt accurately in accord with the process speed at each image forming station and superimpose images in different colors on a paper by adjusting an image forming timing at each image forming station. The process speed referred to here is a rotary speed of a photosensitive drum in a process unit wherein, for instance, a toner image is formed.
In view of this, in case of conventional color copying machines, when a power source of an apparatus is turned ON or in the reset operation after completing this jam processing, test patterns in different colors are formed on a conveyer belt through image forming stations. Based on a timing to detect this test pattern, an amount of misregistration of an image forming position by each image forming station is detected. Based on this amount of misregistration, an image forming timing at each image forming station is adjusted so as to correct the misregistration of a color of image formed on a paper.
As disclosed in U.S. Pat. No. 5,854,958 (Dec. 29, 1998), some of patterns for correcting misregistration are, for instance, wedge shaped patterns that have a first segment of a line extending in the main scanning direction orthogonal to the running direction of a conveyer belt and a second segment extending inclining from one end of this first segment of a line. These wedge shaped test patterns in different colors are formed outside the image forming area in line at equal spaces along the running direction of a conveyer belt, that is, the sub-scanning direction. Then, these test patterns are detected in order through sensors provided opposing to the conveyer belt at the further downstream side than a plurality of image forming stations. Based on these test pattern detecting timings, amount of misregistration of image forming positions by the image forming stations is adjusted and the misregistration of a color is corrected.
However, in said conventional color copying machine, a fixing device is arranged close to one of rollers over which a conveyer belt is put. When affected by the radiant heat from this fixing device, the roller diameter is secularly expanded by heat. If the roller diameter is expanded secularly, the running speed of the conveyer belt gradually increases causing color misregistration on color images that are superimposed on a paper.
That is, when the roller diameter becomes large by, for instance, A mm by the thermal expansion, the length of the outer circumference becomes large by A.times.3.14 mm. As a result, the peripheral speed of the roller, that is, the running speed of the conveyer belt also becomes fast. So, assuming that the change amount of length of the outer circumference of the roller is .DELTA.L, the number of revolutions of the roller until it passes through a plurality of image forming stations is N, the atmospheric temperature rise near the roller is .DELTA.T, a coefficient of thermal expansion of the roller is K, the roller diameter before expansion is R, a distance from the image forming station at the most upper stream side to the image forming station at the most downstream side is D, the maximum value O of the relative misregistration of each color image is: EQU O=.DELTA.L.times.N=.DELTA.T.times.K.times.R.times.D/(R.times..pi.)
a ratio of the misregistration against the degree of expansion of the roller diameter becomes very high.
To prevent the misregistration of colors caused by secular thermal expansion of the roller diameter, for instance, a method to detect and correct the misregistration of colors for each sheet of paper by forming a plurality of wedge shaped test patterns as mentioned above by expanding a space between a preceding paper and a succeeding next paper is considered.
In this method, however, as a space between a preceding paper and a succeeding next paper is expanded, the number of sheets of paper supplied for the image forming in a unit time becomes less and the processing speed of the entire apparatus becomes slow. Further, the sequential process due to the interruption for forming a test pattern will become necessary and the processing speed of the entire apparatus will become further slow. In addition, there may be generated a problem that troubles relative to the sequential process are produced and the quality is lowered.